The measurement of the phase between two sinusoidal signals, or phase detection, has a primary role in a number of signal processing applications.
Phase-Locked Loop (PLL) for example may be used in scanning-probe microscopy applications for example, such as Atomic-Force Microscopy, where the probe is a mechanical crystal or resonator which resonance characteristics change as it gets close to the scanned surface. In such an application, a digital phase-locked loop (PLL), as illustrated for example in FIG. 1, is intended to drive a mechanical crystal or resonator 16 so that its transfer phase is constant, typically of about −90 degrees at resonance. The PLL comprises a variable-frequency oscillator 12 and a phase detector 14. It compares the phase of the input signal from the variable-frequency oscillator 12 with the phase of a signal from the mechanical crystal or resonator 16 and adjusts the frequency of the variable-frequency oscillator 12 to keep the phases matched. The signal from the phase detector 14 is used to control the variable-frequency oscillator 12 in a feedback loop. The phase detector 14 is based on a modulation followed by a low-pass filter operation, i.e. on synchronous demodulation. The role of the low-pass filter is to eliminate upper modulation products at twice the modulation frequency. As a result, the filter limits the useful frequency range of the PLL to at most one octave. In addition the filter must be pre-adjusted according to the intended frequency range of the PLL, which affects both the precision and time response of the phase detector 14. If the filter is wide the frequency range is large and the time response is short, but the precision is poor. Conversely, if the filter is narrow, the frequency range is also narrow, the precision is better, but the time response is poor. So, for a phase detector based on synchronous demodulation, the ratio of precision versus time-response of the phase detector is always tied to the intended frequency range of the PLL.
Other fields, such as, for example digital signal processing, digital radio, software-defined-radio, network analyzers, impedance analyzers, LRC-meters, harmonic synthesis . . . etc. use phase detection.
There is a need in the art for a system and a method for measuring amplitude and phase difference between two sinusoidal signals.
The present description refers to a number of documents, the content of which is herein incorporated by reference in their entirety.